Among display devices such as liquid crystal display devices, there have been developed a touch panel-integrated display device having a touch panel function capable of detecting a position of a panel surface which position is touched by a finger, an input pen, or the like.
Most of conventionally-used touch panel-integrated display devices employ (i) a resistance film type (i.e., a type according to which, in response to being pressed, an upper electrically-conductive substrate and a lower electrically-conductive substrate come in contact with each other, and an input position is detected based on this) or (ii) a capacitive type (i.e., a type according to which a change in capacitance in a touched position is detected and thus an input position is detected). However, such the display device needs, for example, a special panel for position detection. This increases the entire thickness of the device. Further, providing such the touch panel in a screen (display region) of the display device leads to a problem of impairing visibility.
Incidentally, Patent Literature 1 describes a touch sensor system by which a position of the tip of a finger touching a panel is detected by selecting a detection method depending on brightness of natural light as below: (i) If a photo sensor for finger detection provided in the panel detects natural light whose brightness is equal to or greater than a threshold value, a method (finger shadow model) for detecting the shadow of the finger which shadow is made by the natural light is selected. (ii) If natural light detected has brightness which is less than the threshold value, a method (finger cushion reflection model) for detecting light which is emitted from a backlight and reflected by the finger is selected.
Patent Literature 2 describes a touch input device including a display section having upper, lower, right, and left parts each including an array of light-emitting elements and an array of light-receiving elements, wherein an infrared ray is emitted in a direction parallel with a screen of the display section, and a position of the screen of the display section to which position an input is made by a finger or the like is detected based on blocking of the light.